Liquid crystal displays (LCD) have been widely used in various display apparatuses. In some particular applications, such as vehicle-mounted instruments, information displayed on the liquid crystal screen is very important and display on the liquid crystal screen needs to be paid attention to all the time. However, at present, LCD is only used as a video output terminal. When abnormal display occurs, one can only intuitively see the abnormity in display, but cannot determine whether the driving signal of LCD is normal or not and can hardly find the problem.